Field of the Invention
The present invention relates to a lead-free solder alloy which is used for soldering electronic parts; a solder material containing the same; and a joined structure containing the lead-free solder alloy.
Background Art
In recent years, a lead-free solder which contains almost no lead from the viewpoint of an environmental problem is used for mounting electronic parts on an electronic circuit substrate such as a printed wiring board.
On the other hand, the miniaturization of the parts also progresses along with a tendency of the miniaturization of a mount substrate and a wire, and as a result, the miniaturization of a solder joint portion also progresses.
Along with such a tendency of the miniaturization of the solder joint portion, such problems have occurred that a void is formed in the solder joint portion due to the occurrence of electro-migration (electromigration), and further that the wire is disconnected. Then, various technologies for suppressing the electro-migration in the lead-free solder are examined.
For instance, in Patent Literature 1, it is described to provide a protective layer formed of Ag—Sn metal on the surface of a connection terminal. However, when the protective layer is provided on the surface of the connection terminal in this way, it is necessary to change a connection structure itself, and it becomes necessary to reexamine a manufacturing process in a large extent. In addition, a process of providing the protective layer becomes necessary, and the manufacturing process becomes complicated. Then, it has been examined to adjust the composition of the solder alloy and thereby suppress the electro-migration. For instance, in Patent Literature 2, a lead-free solder alloy is described which includes Cu and In of particular contents, and the balance being Sn. In addition, in Patent Literature 3, a lead-free solder alloy is described which includes metal such as Pd, Mn, Zn, Al, Sb and In.
However, in the solder alloys described in the Patent Literatures 2 and 3, an effect of suppressing the electro-migration is insufficient.